When drilling, as in the oil industry, a power tong and backup combination is commonly used to effect a connection between one pipe and another. In order to “makeup” or “break-out” a connection between pipe sections during drilling, it is necessary to hold or clamp the collar or body of the receiving pipe so that it will remain stationary while the other pipe is rotated into engagement with the stationary pipe. This function is done by the use of a “backup tong”, often called simply a “backup”.
Generally, but not exclusively, the backup is located below the main rotary tong. A backup may be C-shaped, fully closed, or initially open prior to gripping, but closed during gripping, as for example in U.S. Pat. No. 5,702,139 (Buck). The backup is normally hydraulically operated (other methods include pneumatic and manual operation) and due to the nature of its function it is desirable that it be as simple as possible in design. The design of a backup generally includes a set of two or more jaws; a means for connecting the backup to the rotary tong; and provision to make a connection to a rigid anchor point in order to absorb the reaction torque on the stationary pipe arising from the torque applied to the rotating pipe by the rotary tong.
Typically, but not exclusively, on a well site a rotary tong/backup assembly is moved into place so that the backup can grasp the stationary pipe. After centering the stationary pipe in the central opening of the backup, the backup is activated to move its jaws into engagement with the pipe. The jaws bite into the pipe to provide sufficient radial force to overcome the tangential force generated by the torque of the rotary tong. Once the stationary pipe is held in place by the backup, the rotary tong provides torque to the rotating pipe in order to make up the joint. The applied torque builds up until the joint is closed. Once the connection is torqued to its final target value, the backup jaws are released from the anchor and retracted from the stationary pipe.
A common form of backup usually includes one or more cylinders (operated either hydraulically or pneumatically), each of which drive a jaw, either directly or indirectly, along a guided slot radially toward the pipe until a sufficient clamping force is attained. This form of backup generally includes a fixed jaw that reacts against this force and serves to center the pipe; along with the aforementioned driving jaw(s).
References herein to “jaws” may be taken as referring to the combined assembly of the jaw carrier and the die that is carried into contact with the pipe, unless the context indicates otherwise.
A number of desired characteristics of a backup tong include: a sufficient clamping force that holds the stationary pipe while torquing; a limit in size and weight to allow the unit to be used on smaller rig floors and/or snubbing baskets; allow the use of jaws which are interchangeable with the tong, thereby reducing the number of spare jaws required to be stocked by the operator; allow the use of wraparound dies; reduce the quantity of parts thereby reducing cost and inventory; optimize the speed with which the jaws engage the stationary pipe while ensuring that the desired maximum torque can be achieved and supported; and generate a controlled radial force required to grip the pipe while minimizing the hydraulic pressure requirement and avoid crushing of the pipe itself.
U.S. Pat. No. 4,494,425 (Shewmak Sr.) discloses a backup tong wherein a moveable jaw is carried at the end of a lever arm that pivots about a pivot point. The end of the lever arm remote from the jaw is actuated by an air cylinder, causing the jaw at the other end to swing in an arc towards contact with a pipe. The pivot axis is eccentric to a curved face of the moveable jaw, so that as the moveable jaw is swung towards the pipe the jaw closes on the pipe. A disadvantage of this configuration is that the jaw, as it advances further towards the pipe once contact is made, tends to move transversely across the face of the pipe.
U.S. Pat. No. 4,402,239 (Mooney) discloses a backup tong wherein the jaw is carried on a jaw carrier that slides radially towards the pipe to be engaged. The jaw carrier is driven into engagement with the drill pipe by one end of a lever that pivots about a pivot point fixed with respect to the tong body, the other end of the lever being coupled to a hydraulically-driven push rod. In this configuration no provision is made for having distinct rates of advancement of the jaw towards the pipe as the lever rotates about its pivot point. Additionally, the radial force generated is purely based on the differential between the pivot point to cylinder attachment, and the pivot point to roller contact point.
In the aforementioned prior art references, the radial force provided by the jaws will vary to some degree, depending on the state of the pipe, which is often oversize or worn.
Accordingly, it would be desirable to provide a backup tong wherein provision is made for having distinct rates of advancement of the jaw towards the pipe as an activating lever is being rotated in order to effect engagement of the jaw with such pipe. It would also be desirable to provide for a design option for controlling the amplification factor for the force originating from an actuator and giving rise to the force being applied by a jaw to the pipe.